1. Field of the Invention
The present invention relates to a radio communication system and an overhang station apparatus, and particularly relates to a radio communication system for optically transmitting a digital baseband radio signal or the like.
2. Description of Related Art
A base transceiver station used for a radio communication system such as mobile phone has a transmission amplifier or an antenna for sufficiently transmitting a radio signal into a base transceiver station cell, in addition to a main function such as call processing to enable radio access from a plurality of subscriber stations. Typically, all of such function means are accommodated in one station building directly under an antenna tower, and particularly in the case that the antenna is installed on a roof of a tall building possessed by a third party, there is a difficulty that when the base transceiver station is also installed in the building, maintenance of the base transceiver station becomes hard, and there is a difficulty that when the base transceiver station is installed in a place suitable for easy maintenance, loss in high frequency cable is increased between the base transceiver station and the antenna.
To overcome such difficulties, a configuration is devised: a radio section such as the transmission amplifier is separated from the base transceiver station, and both are connected by an optical fiber, and standardization including CPRI (Common Public Radio Interface) and OBSAI (Open Base Station Standard Initiative) is underway (for example, see the CPRI standard as non-patent document 1). While the separated radio section is generally called RRH (Remote Radio Head), or called Radio Equipment in CPRI, it is called transmitter and receiver (TRX) amplifier in the specification hereinafter. On the other hand, since the base transceiver station can be installed in an arbitrary place without any restriction, base transceiver stations for a plurality of cells are integrated in one place, consequently they can be installed at reduced cost. Configurations for connecting between the integrated base transceiver station and the distributed TRX amplifiers are roughly divided into Star topology in which the base transceiver station is directly connected to each of the TRX amplifiers, and Chain topology in which the TRX amplifiers are in cascade (daisy chain) connection with one another.
FIG. 1 is a diagram showing a configuration of a usual radio communication system 1, which is supposed to be based on CPRI.
The radio communication system 1 includes a radio network controller (RNC) 100, a base transceiver station 102, TRX amplifiers 2-1 to 2-n (n is an integer of 1 or more), and mobile stations 104-1 to 104-m (m is an integer of 1 or more).
The base transceiver station 102 and the TRX amplifiers 2-1 to 2-n are connected in series via a digital optical fiber line or the like.
Furthermore, the TRX amplifiers 2-1 to 2-n form corresponding cells (sectors) respectively, and connected to the mobile stations 104-1 to 104-m in the cells via radio communication lines.
In the following figures, unnecessary components for description of an embodiment of the invention are appropriately omitted for specific and clear representation.
Furthermore, hereinafter, when a plurality of components such as the TRX amplifiers 2-1 to 2-n are shown without specifying one of them, they may be abbreviated as simply TRX amplifiers 2.
The RNC 100 sets calls in the base transceiver station 102 and other base transceiver stations, and controls such base transceiver stations. Moreover, the RNC 100 transmits and receives user data being wirelessly communicated between the relevant user and the base transceiver station 102.
The base transceiver station 102 receives a signal from the RNC 100, and generates a baseband signal (IQ-data) corresponding to each carrier wave of each cell by a typical function of a base transceiver station, and transmits the baseband signal to the TRX amplifier 2-1. Moreover, the base transceiver station 102 receives IQ-data from the TRX amplifier 2-1, and performs typical processing to the IQ-data and then transmits the IQ-data to the RNC 100.
The TRX amplifier 2-1 relays a signal such as IQ-data between the base transceiver station 102 and the TRX amplifier 2-2.
The TRX amplifier 2-2 relays a signal between the TRX amplifier 2-1 and the TRX amplifier 2-3. After that, similarly, a TRX amplifier 2-i (i is an integer of 2 or more and not more than n−1) relays a signal between a TRX amplifier 2-(i−1) and a TRX amplifier 2-(i+1).
As above, each of the TRX amplifiers 2-1 to 2-n and the base transceiver station 102 transmits and receives a signal to/from each other.
The TRX amplifier 2-i wirelessly receives an uplink signal from the mobile station 104, and performs processing such as amplification, analog to digital conversion, and orthogonal demodulation to convert the signal into IQ-data, and then transmits the IQ-data to the TRX amplifier 2-(i−1), in addition, receives IQ-data to the amplifier 2-i itself from the TRX amplifier 2-(i−1), and performs processing such as digital to analog conversion, orthogonal demodulation of a carrier, and amplification, and then transmits the IQ-data to the mobile station.
Downlink (DL) means signal transmission from the base transceiver station to the mobile station or a direction of the transmission, in addition, the specification uses it as a meaning of a signal transmission direction such as a direction from the base transceiver station 102 to the TRX amplifier 2-1 or a direction from the TRX amplifier 2-1 to the TRX amplifier 2-2 in an arbitrary area between the base transceiver station 102 and the mobile station. This is similar in uplink (UL). Viewing from an arbitrary TRX amplifier, a base transceiver station side is called upstream, and a mobile station side is called downstream.
The mobile stations 104-1 to 104-m transmit and receive signals to/from the TRX amplifiers 2 respectively.
IQ-data transmitted between the base transceiver station and each of the TRX amplifiers 2 is a digital baseband signal at a chip rate in the downlink (DL), and a digital baseband signal at a sample rate twice as high as the chip rate in the uplink (UP).
In both DL and UL, IQ-data for a plurality of TRX amplifiers 2 are accommodated in first to fifteenth words of a basic frame including 16 words. A 0th word (leading word) is allocated to a control word. Each word includes 8 bits or integral multiple of it. One basic frame period is equal to one chip time Tc ( 1/3.84 MHz).
Moreover, a hyper frame is configured with 256 basic frames as a unit. A leading word of a basic frame at a lead of the hyper frame is a synchronous byte (K28.5 code) showing the lead of the hyper frame. Hereinafter, the synchronous byte is called header. Other leading words are used for transmission of a plurality of subchannels (Synchronization and timing, Slow C&M link, Fast C&M link, L1 inband protocol, and Vender specific) by time division multiplex in a unit of hyper frame. Among the subchannels, subchannels except for Vender specific are called C (Control and management) plane data. On the contrary, IQ-data of the first to fifteenth words are called U (User) plane data.
Furthermore, a UMTS Node B frame with 150 hyper frames as a unit is defined.
The frames are converted into serial signals using the 8B10B code, and transmitted through a digital optical fiber line.
Generally, synchronization is often necessary between base transceiver stations configuring cells in a radio access system, and synchronization is indispensable in the case of performing site diversity. The site diversity is applied, for example in the CDMA system, to location service in which a position of a mobile station as a source is estimated from transmission delay when a plurality of base transceiver stations receive a signal from the source.
Therefore, again in the radio communication system 1, it is important that the base transceiver station 102 recognizes and controls processing delay amount between the base transceiver station 102 and each of the TRX amplifiers 2.
For example, “4. 2. 9 Link Delay Accuracy and Cable Delay Calibration” in the CPRI standard as the non-patent document 1 describes a method of adjusting delay in the Star topology and the Chain topology.
FIG. 2 is a diagram showing definition of delay in each section of the radio communication system 1 cited from the non-patent document 1, wherein a case of n=2 is shown.
In the base transceiver station 102, R1 is an output end of the base transceiver station, and R4 is an input end of the base transceiver station.
In the TRX amplifier 2-1, RB2 is an input end of a slave port, RB3 is an output end of the slave port, RB1 is an output end of a master port, and RB4 is an input end of the master port. Here, the master port is a port for outputting a DL signal, and inputting a UL signal (a port seen to be equivalent to R1 or R4 of the base transceiver station for an object to be connected thereto), and the reverse holds in the slave port.
R2 provided in the TRX amplifier 2-2 is an input end of a slave port, R3 is an output end of the slave port, and Ra is an antenna end. Each end is defined in logical connection of a baseband signal (IQ-data) to be transmitted.
Each TRX amplifier 2 is operated based on a clock reproduced from a frame signal inputted into R2.
T12(1) is delay amount from R1 of the base transceiver station 102 to RB1 of the TRX amplifier 2-1, and T12(2) is delay amount from RB1 of the TRX amplifier 2-1 to R2 of the TRX amplifier 2-2.
TBdelayDL(1) is delay amount from RB2 to RB1 of the TRX amplifier 2-1, and T2a is processing delay amount from R2 to Ra of the TRX amplifier 2-2.
T34(1) is delay amount from the output end RB3 of the TRX amplifier 2-1 to R4 of the base transceiver station, and T34(2) is delay amount from R3 of the TRX amplifier 2-2 to RB4 of the TRX amplifier 2-1.
TBdelayUL(1) is processing delay amount from RB4 to RB3 of the TRX amplifier 2-1, and T3a is processing delay amount from Ra to R3 of the TRX amplifier 2-2.
T14(1) is time difference between a header (frame timing) of output and a header of input at a master port end of the base transceiver station.
Toffset is time difference between a header inputted from R2 and a header outputted from R3 of the TRX amplifier 2-2, and set so as to be substantially equal to total time of processing delay (T2a) at a DL side and processing delay (Ta3) at a UL side of the TRX amplifier 2-2.
Toffset(1) is also set so as to be substantially equal to total time of processing delay (delay from RB2 to the antenna) at a DL side and processing delay at a UL side of the TRX amplifier 2-1.
FIG. 3 is a frame timing chart of DL and UL of the radio communication system 1 cited from the CPRI standard as the non-patent document 1.
T12 is delay in a period from R1 of the base transceiver station 102 to R2 of the terminal TRX amplifier 2-2, andT12=T12(1)+TBdelayDL(1)+T12(2) is given.
T34 is delay in a period from R3 of the terminal TRX amplifier 2-2 to R4 of the base transceiver station 102, andT34=T34(1)+TBdelayUL(1)+T34(2) is given.
T14 is delay in IQ-data in a period from a point when the IQ-data are outputted from R1 of the base transceiver station 102 to a point when they are returned to R4 via the TRX amplifier 2-2, andT14=T12+Toffset+T34 is given.On the other hand, T14(1) being delay in frame timing observed in the base transceiver station 102 is different from T14 because Toffset(1) is determined in the inside of the TRX amplifier 2-1 irrespective of delay in downstream, andT14(1)=T12(1)+Toffset(1)+T34(1) is given.
BFN is a UMTS Node B frame number, and HFN is a hyper frame number. The TRX amplifiers 2 essentially use BFN and HFN received in DL as they are in UL.
As shown in the lowest frame timing in FIG. 3,T14−T14(1)=Tc×N(1) (N(1) is an arbitrary integer)is obtained in the CPRI standard.That is, IQ-data from the TRX amplifier 2-2 may be accommodated in a basic frame, which is n frames later than the same basic frame as that of the TRX amplifier 2-1, rather than the same basic frame.
As expressed by the following expression, T14−T14(1) is difference between roundtrip time from an input end to an output end of the slave port of the TRX amplifier 2-1 via the TRX amplifier 2-1, and Toffset(1) in the inside of the TRX amplifier 2-1, and called roundtrip time difference hereinafter.T14−T14(1)=TBdelayDL(1)+T12(2)+Toffset(2)+T34(2)+TBdelayUL(1)−Toffset(1) When all of TBdelayDL(i), TBdelayUL(i), Toffset(i), Toffset(i+1), and N(i) are known in the base transceiver station, T12(i+1)+T34(i+1) can be calculated from them. That is,T12(i+1)+T34(i+1)=Tc×N(i)+Toffset(i)−(TBdelayDL(i)+Toffset(i+1)+TBdelayUL(i)) is given.Moreover, since an assumption of T12(i+1)=T34(i+1) is made with a sufficiently small error compared with Tc, T12(i+1) and T34(i+1) can be individually obtained.
While T12 or T34, which is delay in an optical cable, is large compared with other processing delay, and may reach to a few hundred of chips, it is not ensured to be in a unit of chip. Moreover, uncertainty in delay exists in a Serdes device while it is slight.
In addition, as a related art on an embodiment of the invention, JP-A-2-174428 discloses a delay adjustment method in which sending timing of signals, which are transmitted by a plurality of radio base transceiver stations in cascade connection, to a control station is made equal in all the radio base transceiver stations.
Moreover, JP-A-7-298347 discloses a method in which a central control station to be connected to a plurality of radio base transceiver stations performs control by estimating transmission delay amount based on phase difference between an uplink frame and a downlink frame.
Patent document 1: JP-A-2-174428.
Patent document 2: JP-A-7-298347.